The field of endeavor generally relates to means of mounting a carrier rack on a vehicle and particularly concerns mounting on a tow-ball that is securely attached to or integral with a tow-bar.                A variety of mechanisms have been adopted to fulfill the objective of providing a rigidly located member on a tow-bar that has the tow-ball attached. The previously adopted mechanisms fall into one of two categories:        1) Category 1. The tow-ball is only used to bolt the member or the base-plate for a member to the tow-bar. In the case of it being a base-plate the member is then rigidly linked to the base-plate by a variety of methods that would be obvious to anyone skilled in the art. With this category the stability of the member is not dependent on any direct engagement between the member and the head of the tow-ball.        2) Category 2. The head of the tow-ball is critical to establishing a rigidly located member. Thus far this approach has involved surrounding the head of the tow-ball to a varying degree and using friction to grip the head of the tow-ball. Various means have been described to establish a sufficient compression force to create adequate friction between the member and the tow-ball head to avoid the member slipping from its desired position when subjected to a displacing force. In many instances generating the gripping (compression) force requires a specific tool. U.S. Pat. No. 5,228,607 and New Zealand Patent number 226926 are relevant to the concept of using friction between a tow-ball head and a member to retain the member in a desired position.        
Both of the above categories of attachment suffer from the possibility of the member moving substantially, relative to the tow-bar, if the securing-nut on the stem of the tow-ball is not sufficiently tight. In both categories the member can separate completely from the tow-bar if the securing-nut on the stem of the tow-ball comes completely off. An object separating from a tow-bar represents a significant hazard to other road users.
Category 1 mechanisms that are bolted directly to the tow-bar also suffer from the disadvantage of having to unbolt then re-attach the tow-ball each time the device is to be removed or attached.
Category 1 mechanisms that use a base-plate have the advantage of convenience of removal or attachment of the remainder of the member without having to unbolt the tow-ball. Previously disclosed base-plate designs however present certain shortcomings that relate to their type of configuration. There have been two broad types of configuration adopted:
a) a vertical component that extends upwards in close proximity to the tow-ball wherein the vertical component obstructs a ball-coupling articulating with the tow-ball. The obstruction occurs because there is generally insufficient space to locate such a vertical component between the tow-ball and the rear of the vehicle where it would not hinder the articulation of a ball-coupling.
b) a horizontal component that extends for quite a distance to one or both sides of the tow-bar so that vertical elements arising from it cause minimal interference with the range of articulation available to a ball-coupling linked to the tow-ball that secures the base-plate. These bulky horizontal plates represent an impediment to standing behind the vehicle to access the boot (trunk). Such sideways extensions from the tow-bar are also unsightly and a potential hazard to pedestrians.
Category 2 devices invite the risk of the member slipping on the tow-ball if the friction between the tow-ball and the device is inadequate. The friction may be inadequate because the compression force exerted on the tow-ball is inadequate at the outset or declines during use. Friction between the tow-ball and the member is also lowered if the tow-ball is lubricated, as is often the case following use of the tow-ball for towing.